Many industrial plants in the pulp and paper industry and the petro-chemical industry have a utility part which is a multiunit power plant for providing both electrical energy and process steam by cogeneration. The objective of this utility part of the industrial plant is to load the boilers, the turbine generators, and the tie lines supplying electrical power according to cost, consistent with system constraints and the desired controllability of the header pressure controls. Thus, for example, where a backpressure and an extraction turbine are operating in parallel to maintain a header pressure, it is desirable to divide the steam demand for the particular header being supplied so that the most economic division of load between the two turbines is accomplished while maintaining the desired controllability on the header pressure. In some cases the cost curves for the turbines supplying the header overlap and equal incremental cost loading may be used to arrive at an economic division of the load. However, for many combinations of back-pressure and extraction turbines there is no overlap of the cost curves and another approach must be used.
It is therefore an objective of this invention to provide a method and means for dividing the steam load on a header between backpressure and extraction turbines which will provide an economic division of the steam load while maintaining controllability on the header pressure control.
In accordance with the above object there is provided a control system for turbines in the utility part of an industrial plant using cogeneration which control system is operable to control the relative quantities of steam supplied from a plurality of steam turbines such as a backpressure and an extraction turbine to a common steam header which supplies process steam at a predetermined pressure while the electrical generation from the generators driven by said turbines is maintained at a maximum consistent with the maintenance of an adequate control range for the header pressure control on those turbines assigned to maintain header pressure, usually the extraction turbines supplying the header. The system includes means for measuring the header pressure combined with means for controlling the steam flow from a turbine such as an extraction turbine so as to maintain the header pressure at a set point. The system also includes means for measuring the steam flow to the header from the turbine being controlled to maintain the header pressure at its desired value as well as means for controlling the steam flow from the other turbines supplying the header to maintain a predetermined relationship between the steam flows from said turbines.